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| United States Patent |
5,624,488
|
|
Forbus
, et al.
|
April 29, 1997
|
Ultrahigh brightness calcined clay pigment, manufacture & use thereof
Abstract
A selected kaolin crude or crude fraction is purified by physical or
physical/chemical means, such as froth flotation and/or selective
flocculation, to remove discrete particles of TiO.sub.2 and in some cases,
discrete iron minerals. The pure (or purified clay) must then be agitated
in the presence of water with a particulate grinding media such as, for
example, sand, alumina or zirconia beads, to increase the 2 micron content
of the kaolin. An intermediate ground product that contains a substantial
weight percentage of particles finer than 1 micron is generated as a
result of grinding. The particles finer than 1 micron in the ground kaolin
also include those particles in the previously pure or purified kaolin
that were present in the naturally occurring clay. A pulp of the ground
clay is then fractionated to remove particles larger than 2 microns, e.g.,
to remove at least 95%, preferably at 100% by weight, of the particles
larger than 2 microns, while minimizing the removal of particles finer
than 1 micron. The resulting fine particles size fraction is then treated
by conventional series of steps, i.e., optional bleaching, drying,
pulverization, calcination and repulverization to produce a low abrasion,
ultrahigh brightness (typically 96%) calcined kaolin pigment.
| Inventors:
|
Forbus; Ellen S. (Gray, GA);
Suitch; Paul R. (Milledgeville, GA);
Dombrowski; Thomas (Macon, GA)
|
| Assignee:
|
Engelhard Corporation (Iselin, NJ)
|
| Appl. No.:
|
497241 |
| Filed:
|
June 30, 1995 |
| Current U.S. Class: |
106/484; 106/486; 162/181.1; 162/181.8; 209/162; 241/22; 241/23; 241/24.1; 241/29; 428/537.5; 501/145 |
| Intern'l Class: |
C04B 014/10 |
| Field of Search: |
106/484,486
501/145
241/22,23,24.1,29
209/162
162/181.1,181.8
428/537.5
|
References Cited
U.S. Patent Documents
| 2990958 | Jul., 1961 | Greene et al. | 209/166.
|
| 3014836 | Dec., 1961 | Proctor | 162/181.
|
| 3058671 | Oct., 1962 | Billue | 241/24.
|
| 3171718 | Mar., 1965 | Gunn et al. | 100/416.
|
| 3343973 | Sep., 1967 | Billue | 106/486.
|
| 3519453 | Jul., 1970 | Morris et al. | 106/486.
|
| 3586523 | Jun., 1971 | Fanselow et al. | 106/486.
|
| 3743190 | Jul., 1973 | Whitley | 241/4.
|
| 4246039 | Jan., 1981 | Mixon, Jr. | 106/484.
|
| 4381948 | May., 1983 | McConnell et al. | 106/288.
|
| 4492628 | Jan., 1985 | Young et al. | 209/5.
|
| 4738726 | Apr., 1988 | Pratt et al. | 106/487.
|
| 5011534 | Apr., 1991 | Berube et all | 106/416.
|
| 5137574 | Aug., 1992 | Suitch et al. | 106/439.
|
| 5371051 | Dec., 1994 | Pope et al. | 501/145.
|
| 5393340 | Feb., 1995 | Slepetys et al. | 106/484.
|
| 5454865 | Oct., 1995 | Ginn et al. | 106/486.
|
Primary Examiner: Bell; Mark L.
Assistant Examiner: Marcheschi; Michael
Attorney, Agent or Firm: Moselle; Inez L.
Claims
We claim:
1. An improved process for producing an ultra high brightness, low abrasion
kaolin clay pigment from a soft kaolin clay crude which comprises:
a) selecting a crude that contains less than 2.0% by weight of quartz, mica
and smectite minerals in the minus 2 micron fraction, removing grit from
said crude and recovering degritted unfractionated crude that is from
about 50-65% by weight finer than 2 microns, e.s.d.;
b) subjecting the degritted unfractionated crude to froth flotation,
selective flocculation or combinations thereof to remove colored titania
and optionally ferruginous impurities and produce a purified
unfractionated kaolin product containing from 0-1.8% TiO.sub.2 and 0-0.8%
Fe.sub.2 O.sub.3 ;
c) agitating the product of step (b) with particulate grinding media until
about 85-92% by weight is finer than 2 microns e.s.d.;
d) removing sufficient kaolin particles from the product of step (c) to
produce calciner feed that is about 95-100% by weight finer than 2 microns
and about 88-92% by weight finer than 1 micron and represents at least 40%
by weight of the degritted kaolin; recovering a resulting fractionated
product;
e) spray drying the resulting recovered fractionated product from step (d)
and pulverizing, fully calcining and repulverizing the spray dried product
in a conventional manner;
f) and recovering the calcined product having a G.E. brightness of at least
95%, an Einlehner abrasion below 25 and a white color as measured by an L
value greater than 98.
2. An improved process for producing an ultra high brightness, low abrasion
kaolin clay pigment from a soft kaolin crude which comprises:
a) selecting a soft kaolin clay crude that contains less than 2.0% by
weight of quartz and mica in the minus 2 micron fraction thereof,
b) removing grit from said crude,
c) fractionating the degritted crude and recovering a fine size fraction
thereof that is about 60 to 85% by weight finer than 2 microns, e.s.d.,
d) subjecting the recovered fraction from step (c) to flotation, selective
flocculation or combinations thereof to remove colored impurities and
recovering the fraction of clay that contains from 0-0.8% TiO.sub.2 and
from 0-0.8% Fe.sub.2 O.sub.3,
e) agitating the fraction of kaolin with particulate grinding media until
from 95-100% by weight is finer than 2 microns, e.s.d.,
f) removing sufficient particles larger than 2 microns e.s.d. from the
fraction of step (e) to result in calciner feed that is 90-95% finer than
1 micron, e.s.d., and represents at least 50% by weight of the degritted
crude from step (b); recovering a resulting fractionated product;
g) spray drying the recovered product of step (f), pulverizing, fully
calcining and repulverizing in conventional manner and;
h) recovering the calcined product having a GE brightness of at least 95%,
and Einlehner abrasion below 25 and a white color as measured by an L
value greater than 98.
3. A fully calcined kaolin pigment having a GE brightness of at least 95%,
an Einlehner abrasion below 25, a white color, and pore volume of 1-1.3
cc/g.
4. The pigment of claim 3 which has a GE brightness of 95.5% or higher.
5. The pigment of claim 3 which has a GE brightness of 96.0% of higher.
6. Paper coated with the kaolin product of claim 3.
Description
FIELD OF THE INVENTION
This invention relates to calcined, low abrasion, high brightness calcined
kaolin clay pigments, their manufacture from selected soft kaolin crudes
and the use thereof in the manufacture of paper products.
BACKGROUND OF THE INVENTION
One of the seminal developments in the industrial minerals field was the
discovery that calcination of a particular type of poorly crystallized,
ultrafine tertiary kaolin clay, known in the art as "hard" kaolin,
resulted in bright, e.g., 93.5% GE brightness, pigments having desirable
low abrasion and outstanding potential to provide opacity to filled and
coated paper products. Reference is made to U.S. Pat. No. 3,586,523,
Fanselow et al (1971). Products made in accordance with the teachings of
this patent by the assignee and its licensees have accounted for a
substantial proportion of the sales of high performance industrial
minerals in the United States and abroad. Aside from possessing a unique
combination of high brightness, low abrasion and opacification potential,
the processing was remarkably economical because of the unique morphology
as well as inherently fine particle size and desirable particle size
distribution of the kaolin in the crude. In such crudes, colored
impurities, in particular iron-contaminated titania particles, concentrate
in the coarse fraction of the crude. Thus, conventional degritting and
fractionation to recover calciner feed yield a fine fraction of hydrous
kaolin with a lower content of colored titania than the crude. As a result
of these factors, the yield of calcined kaolin product was high, e.g.,
roughly about 50% based on the weight of degritted crude or about 40% by
weight of the particles finer than 1 micron in the degritted crude.
Unlike the hard Georgia Tertiary kaolins that are poorly crystallized and
have an average particle size of about 0.3 microns, (equivalent spherical
diameter or e.s.d.), the soft kaolins that are present in even greater
abundance in Georgia, U.S.A. and throughout the world are composed of much
larger, well crystallized kaolin particles having an average particle size
of approximately 0.6 microns, e.s.d., roughly twice that of the hard
kaolins. Fractionation of a soft clay to isolate the fine size fraction of
the crude having generally the same average particle size and particle
size distribution of a typical degritted hard kaolin would represent only
about a 15% yield (or 85% loss) of degritted kaolin. Thus, assuming that
calcination of such a fine fraction of a degritted soft kaolin would
result in a bright, low abrasion opacifying pigment, at least comparable
in all significant performance characteristics to calcined pigments
prepared from hard clay, the processing would not be economically viable.
It is well known in the industry that the brightness of uncalcined and
calcined kaolin pigments is adversely affected by the presence of colored
impurities, notably titaniferous and ferruginous impurities. It has long
been the practice to remove these impurities to various extents by
physical or physical-chemical means, such as froth flotation, selective
flocculation, magnetic purification, bleaching and combinations thereof.
It is also known that calcination generally increases brightness of a
hydrous kaolin when the clay is "fully" calcined, i.e., calcined to
undergo the characteristic exotherm, and may decrease in brightness when
the clay is calcined under less severe conditions, i.e., to so-called
"metakaolin" state. Thus, 90% brightness (fully) calcined pigments can be
produced readily from hard crudes by degritting, fractionation, bleaching
and calcination; 93% brightness pigments can be obtained from the same
crudes by adding one or more steps to remove colored impurities,
especially titaniferous matter. The Fanselow et al patent (supra)
discloses 95% brightness calcined kaolin pigments.
Attempts were recently made to produce calcined pigments having much higher
brightness from the hard kaolin crudes that are readily processed to
provide 93+% brightness calcined kaolin pigments. To the best of our
knowledge, there is at present no means to produce calcined kaolin
pigments having a 95+% brightness merely by decreasing the content of
colored impurities in the hard kaolin by conventional means such as
flotation.
On the other hand, there are reports in the patent literature dating back
to the early '60's of the recovery of calcined kaolin pigments having 96%
brightness and higher from certain Georgia kaolins crudes. These crudes
are obviously of the soft type as evidenced by information about particle
size and particle size distribution in the patents. Reference is made to
the following:
U.S. Pat. No. 3,058,671 (1962) Billue
U.S. Pat. No. 3,343,973 (1967) Billue
U.S. Pat. No. 3,171,718 (1965) Gunn, et al
Billue allegedly discovered that media grinding of the coarse fraction of
kaolins resulted in "fracture-induced" brightness and found that the
enhanced brightness was carried over when the fractured kaolin was fully
calcined. Specifically, Billue isolated from a naturally occurring crude a
coarse fraction that contained not more than 35% by weight of particles
finer than 2 microns, i.e., a coarse fraction of crude containing at least
65% by weight of particles larger than 2 microns. In Example II (both
patents), a coarse fraction of kaolin was "fractured" to produce a
fractured clay having an initial brightness of 89.1%. This hydrous kaolin
was calcined to produce a product having a brightness of 96.3% (col.7).
However, data in the patent reveal that the portion of the clay that was
fractured (the 86% minus 2 micron cut) represented a meager 28.8% by
weight of the crude. Thus, while calcined brightness was very high, the
process depended upon the removal of minus 2 micron fines before grinding
and would be commercially useless unless the higher brightness would
compensate for the high cost associated with low yield. Billue did not
report the abrasion value or provide information about the opacification
potential of his 96.3% brightness calcined clay. Gunn et al teach media
grinding of coarse filler clay in the production of calcined kaolin
pigments made by mechanical delamination of a coarse hydrous filler clay
fraction. Disclosed is a product having a GE brightness value of 95% and
"low" abrasion as obtained by a specific modification of a Valley Abrasion
test. The clay was delaminated gently to separate kaolin booklets (as
contrasted with severe grinding to break particles after delaminating the
clay). In an example, delaminated kaolin having a GE brightness of 92.1%
was calcined to a GE brightness of about 96%. Gunn et al do not report
data such as to permit calculation of yield, however, the teaching of
Billue (supra) provide guidance and suggest yield would be low.
The Gunn et al patent also clearly teaches that the finest fraction of the
kaolin has the highest content of TiO.sub.2 impurity (Example 9). In other
words, in this type of kaolin, colored impurities concentrate in the fine
end. This is well known to clay mineralogist and the clay industry. It is
noted that Gunn et al did not disclose calcining the fine fraction.
However, based on the disclosure as to the concentration of colored
impurities in the fine end of soft kaolins, one would avoid the presence
of excessive fines if seeking brightness from a purified kaolin obtained
from a soft kaolin crude. This is consistent with the specific disclosure
in the Gunn et al patent of ultrahigh brightness calcined pigments made by
processing the coarse fraction of a crude after the removal of fines.
In U.S. Pat. No. 5,137,574, Suitch et al, a high opacifying pigment is
produced by calcining one or more high titania content kaolin fractions
which are separated from a whole crude by size classification, froth
flotation, magnetic separation or the like. In embodiments of the
invention, a grinding step is practiced before calcination. The grinding
step is said to be necessary in order to reduce the particle size by
virtue of the use of a titania enriched product. The calcined product has
a brightness in the range of about 70 to 82% and the titania content in
greater than 2%. The low brightness product is supplied under the
registered trademark OPACITEX. Production of such a pigment by selection
of a low brightness kaolin crude is described in U.S. Pat. No. 5,371,051,
Pope et al.
McConnell et al, U.S. Pat. No. 4,381,948, now commonly assigned, discloses
a process for producing high brightness, low abrasion calcined kaolin
pigments having exceptionally high light scattering (superior
opacification), a GE brightness of at least 93% and low abrasion (Valley
Test Method) when used as a paper filler. Processing involves use of a
crude clay containing not more than 0.5% glass forming oxides, not more
than 1.5% Fe.sub.2 O.sub.3 and more than 2% TiO.sub.2 and separating a
very fine particle size fraction from such crude, i.e., a fraction that is
100% by weight finer than 1 micron, followed by conventional steps of
drying, pulverizing, calcining and repulverizing. In all illustrative
examples, a hard kaolin crude was processed. The highest brightness
disclosed was 94.3%. The patent also teaches that soft Georgia kaolin
crudes (identified as crudes that are 50-60% finer than 2 microns) can be
used. However, the inventors caution that in such case the particular
crude should contain sufficient 1 micron particles to enable "worthwhile
recoveries". Teachings sufficient to suggest, much less to enable, the
achievement of worthwhile recoveries are not presented in making the
disclosure as to the usefulness of soft kaolins.
Moreover, patentees did not deal with the problem inherent in the use of a
very fine fraction when using soft kaolin because of the concentration of
colored impurities, especially colored titania, in the finest fraction of
soft kaolins.
An early patent in the art, U.S. Pat. No. 3,014,836, Proctor, discloses
that the particle size of a calcined kaolin product depends on the
particle size of the uncalcined kaolin feed and that the feed should be
free from abrasive impurities to produce low abrasion, calcined clay. The
highest GE brightness value was 93%.
U.S. Pat. No. 3,519,453, Morris et al, mechanically delaminate booklets of
a coarse kaolin and calcine to metakaolin conditions in an effort to
obtain low abrasion; GE brightness was below 90%. Slepetys et al, U.S.
Pat. No. 5,393,340, commonly assigned, severely grind booklets in a coarse
kaolin fraction so as to obtain a ground material, the particles of which
are substantially all finer than 1 micron as a result of grinding.
Patentees then calcine to metakaolin to obtain ultralow Einlehner
abrasion; brightness was about 90%. Thus, ultrahigh brightness calcined
kaolin, 95-96% and above, has been reported as has calcined kaolins
characterized by low abrasion (by various methods including the now
obsolete Valley method), high opacification or combinations thereof. Prior
to this invention, however, brightness values above 95% have been achieved
only by processing fractionated soft kaolin crude by steps that resulted
in economically unattractive yields and do not inherently provide calcined
kaolin pigments that also possess sufficiently high light scatter for
paper use combined with the extremely low abrasion values now demanded by
the paper industry. On the other hand, high yields have obtained when
using hard crudes but 96% brightness products were not produced. Grinding
and/or delamination prior to calcination have been advocated with kaolin
but not in a process that results in pigments brighter than 95% at high
yields, for example a recovery in excess of 45%, based on the weight of
particles finer than 1 micron in the crude.
SUMMARY OF THE INVENTION
We have invented a simple method for processing kaolin clay that results in
a calcined pigment having both low abrasion, ultrahigh GE brightness (at
least 95.0%, preferably at least 95.5%, most preferably at least 96.0%),
and high light scatter, using a novel combination of steps that results in
remarkably high yields of the calcined product.
Our method features the selection of a degritted soft kaolin crude that
contains from about 50 to 60% by weight of particles finer than 2 microns
or, preferably, a fraction of degritted soft kaolin crude that is from
about 60 to 85% by weight finer than 2 microns, the crude or crude
fraction possessing the following combination of properties: iron content
(as Fe.sub.2 O.sub.3) below 0.80%, preferably below 0.60 wt. %, and most
preferably below 0.45 wt. %, and TiO.sub.2 below about 1.80 wt. %,
preferably below 0.75 wt. % and most preferably below about 0.50 wt. %.
The minus 2 micron component of the degritted material contains less than
a total of 2.0% preferably, less than 1.0%, and most preferably less than
1.0% of any of the following impurities: mica, quartz, cristobalite (or
other forms of silica), or smectite (montmorillonite) minerals; less than
about 1% by weight of the selected degritted crude or crude fraction is
composed of particles larger than 325 mesh (U.S. Sieve).
Unless the TiO.sub.2 content of selected crude or crude fraction is below
1%, it must then be purified by physical or physical/chemical means,
preferably by froth flotation, high intensity magnetic separation (HIMS),
selective flocculation or combinations thereof, to remove discrete
particles of TiO.sub.2 and in some cases, discrete iron minerals. The pure
(or purified clay) must then be agitated in the presence of water with a
particulate grinding media such as, for example, glass beads, sand,
alumina or zirconia beads, to increase the 2 micron content by at least
20%, preferably 25 to 30%, in the case of a degritted crude or at least
10%, and preferably from about 15 to 20% in the case of fractionated
crude, and to result in a ground material that is at least about 75%,
preferably 80 to 87%, finer than 1 micron. Thus, an intermediate ground
product that contains a substantial weight percentage of particles finer
than 1 micron is generated as a result of grinding. The particles finer
than 1 micron in the ground kaolin also include those particles in the
previously pure or purified kaolin that were present in the naturally
occurring clay. A pulp of the ground clay is then fractionated to remove
particles larger than 2 microns, e.g., to remove at least 95%, preferably
100% by weight, of the particles larger than 2 microns, while minimizing
the removal of particles finer than 1 micron. The resulting fine particles
size fraction is then treated by conventional series of steps, i.e.,
optional bleaching, drying, pulverization, calcination and
repulverization.
In one embodiment of the invention (using degritted but unfractionated
coarse soft kaolin), the process comprises the following steps;
a) providing a degritted soft kaolin crude that is from 50-65% by weight
finer than 2 microns, e.s.d. and contains substantially all the minus 1
micron particles present in the degritted naturally occurring clay, the
degritted crude containing from 0 to 1.8% by weight TiO.sub.2 and 0 to
0.8% by weight Fe.sub.2 O.sub.3 based on the dry weight the degritted
crude, and having low levels of mica, quartz or other silica minerals and
montmorillonite minerals;
b) removing colored impurities by selective flocculation, froth flotation
or combinations thereof to reduce the titania content thereof to a level
below 1% if the titania content exceeds 1% in step (a);
c) agitating the degritted kaolin clay in the presence of water with
particulate grinding media until the particle size is from 85-92% by
weight finer than 2 microns;
d) removing sufficient particles larger than 2 micron from the product of
step (c) by sedimentation or centrifugation to produce a product that is
about 95-100% by weight finer than 2 micron and about 88-92% finer than 1
micron and to provide calcined feed that is at least 40% by weight of the
clay from step (a); preferably bleaching the fractionated clay;
e) spray drying the recovered product of step (d) pulverizing, fully
calcining and repulverizing in conventional manner and;
f) recovering the calcined product having a GE brightness of at least 95%,
and Einlehner abrasion below 25 and a white color. (L value greater than
98)
In preferred embodiment, the improved process for producing an ultra high
brightness, low abrasion kaolin clay pigment from a soft kaolin crude
comprises:
a) selecting a soft kaolin clay crude that contains less than a total of 1%
by weight quartz or other crystalline silica minerals, mica, and smectite
minerals in the minus 2 micron fraction thereof, the crude containing less
than 0.8% Fe.sub.2 O.sub.3 and less than 1.8% TiO.sub.2 after removal of
grit in step (b),
b) removing grit from said crude,
c) fractionating the resulting, degritted crude and recovering a fine size
fraction thereof that is from 60 to 85% by weight, preferably about 76% to
82%, e.g., about 80% by weight, finer than 2 microns, e.s.d.,
d) subjecting the resulting fraction from step (c) to flotation, selective
flocculation or combination thereof to remove colored impurities and
recovering a beneficiated fraction of clay that contains from 0-0.8%
TiO.sub.2 and from 0-0.8% Fe.sub.2 O.sub.3,
e) agitating the beneficiated fraction of kaolin with particulate grinding
media until from 95-100% by weight is finer than 2 microns, e.s.d.,
f) removing sufficient particles larger than 2 micron e.s.d. from the
product of step (e) to result in calciner feed that is about 90-95% finer
than 1 micron, e.s.d., and represents at least 50% by weight of the
degritted crude from step (b); optionally bleaching,
g) spray drying the recovered product of step (f);
h) pulverizing, fully calcining and repulverizing the product of step (g)
in conventional manner and;
i) recovering the calcined product having a GE brightness of at least 95%,
an Einlehner abrasion below 25 and a white color--(L value of at least
98).
The latter embodiment of the process is preferred because it permits a
significantly greater recovery of clay suitable as calciner feed. This
improved recovery results from the fact that more of the minus 1 micron
particles generated during grinding are recovered when operating with feed
that is fractionated to remove some plus 2 micron particles prior to
grinding. The removal of the coarse fraction prior to delamination
improves the yields and economics of the grinding step.
Calcined kaolin pigments of the invention have a G.E. brightness of at
least 95% (typically 95.5-96.8%), an Einlehner abrasion below 27,
preferably below 22 mg, typically 19 to 21, and light scatter as
determined by the known black glass test procedure of at least 0.2 m.sup.2
/g usually 0.312 to 0.317 m.sup.2 /g. The pigments have a white color,
with a more blue shade than commercial 93 brightness calcined kaolins
produced from hard crudes. Typical color values are: L*=97.44, a*-0.58 and
b*-2.84. Particle size is typically 97-100% minus 5 microns, 84-88% minus
2 microns and 55-65% minus 1 micron.
The interaggregate pore volume is from 0.9 to 1.4 cc/g at a pore radius of
2000 to 4000 Angstrom units, preferably 1.0 to 1.3 cc/g, at a radius of
2500 to 3500 Angstrom units, as measured by Mercury Porosimetry.
Calcined kaolin pigments of the invention are useful in coating and filling
a variety of paper and paperboard products. The pigments are also useful
as extender pigments in paints, plastic and rubber. Presently preferred
best modes of use is as a filler for uncoated specialty papers such as
premium/specialty printing paper and recycled fiber containing printing
papers. In such use, the pigments are a cost effective alternative to
hydrated alumina and precipitated silicates.
DETAILED DESCRIPTION
It is well known in the art that when kaolin clay is calcined, it undergoes
a series of characteristic changes, detectable by differential thermal
analysis (DTA). At about 840.degree.-1200.degree. F.
(450.degree.-650.degree. C.), the clay undergoes a strongly endothermic
dehydration reaction resulting in the conversion to material known as
metakaolin. The metakaolin state is conveniently ascertained by acid
solubility testing because the alumina in the clay is virtually completely
soluble in strong mineral acid. Typically, about 45% by weight of
metakaolin is soluble in hydrochloric acid of 18% strength. In contrast,
solubility in hydrochloric acid of the alumina component in hydrated
kaolin is very limited. Furthermore, when kaolin is calcined beyond the
endotherm at higher temperatures it undergoes a characteristic exothermic
reaction, resulting in phase transformation manifested by markedly reduced
alumina solubility. Upon further calcination, mullite is crystallized.
Formation of mullite is avoided in practice of this invention because
mullite is abrasive.
Calcined kaolin pigments have been used for several decades in a number of
industrial applications such as paper coating, paper filling, paints,
plastics, etc. In these applications they impart to the finished products
a number of desirable properties: brightness, opacity, hiding power,
strength (in plastics), friction (in paper). Paper coating and filling
applications require almost exclusively fine fully calcined kaolin
pigments such as the 93% brightness ANSILEX 93.RTM. pigment manufactured
by Engelhard Corporation. See, for example, U.S. Pat. No. 3,586,523,
Fanselow et al, which describes the production of such pigments from
ultrafine Tertiary "hard" ultrafine kaolins. Because of high brightness
and light scattering properties of these fine fully calcined kaolin
pigments, their primary function in paper applications is to provide
opacity and brightness, often as a replacement for much costlier titanium
dioxide pigments, which can also be used to enhance these functional
properties.
Although these fully calcined kaolin pigments obtained by calcining
ultrafine hard kaolins are less abrasive than other calcined kaolin
pigments, they are relatively abrasive when compared with available
noncalcined kaolin pigments. For example, the conventional so-called "low
abrasion" calcined kaolin pigments, such as the pigment supplied under the
registered trademark ANSILEX 93, typically have an Einlehner abrasion
value of about 20 mg. The Einlehner abrasion test is now practiced widely
by the industry and has replaced the older Valley test. In practical
terms, high abrasion translates into increased wear of bronze web forming
screens (wires) on paper making machines, dulling of paper slitter knives,
wear of printing plates when they come in contact with coated paper
containing fine calcined pigments in the coating formulation, and, in
general, wear of any surface that comes in contact with these pigments.
Paper makers are becoming increasingly demanding in their need for lower
abrasion.
Brightness of calcined kaolin pigments is very strongly influenced by
discoloring contaminants. The two most important ones in kaolin pigment
technology are iron and titanium oxides. Typically, fully calcined kaolin
pigments which are produced from fine hard Middle Georgia Tertiary kaolin
crudes, such as those mentioned in U.S. Pat. No. 3,586,523, carry iron and
titanium contamination of about 0.90-1.1% Fe.sub.2 O.sub.3 and 1.0-1.8%
TiO.sub.2, respectively. While the role of colored impurities in the
brightness of calcined kaolin pigments is recognized, prior to this
invention those skilled in the art were not successful using this
knowledge alone to introduce to the market calcined kaolin pigments with
ultrahigh brightness (e.g., 96 GE % brightness or above), in combination
with low abrasion and good opacification potential.
This invention uses grinding of coarse kaolin booklets as opposed to
delamination to achieve valuable fully calcined kaolin pigments. The terms
"delamination" and "grinding", as used herein, are distinguishable. Soft
clay deposits contain naturally separated platy kaolin particles as well
as "booklets", which comprise stacks of kaolin platelets. These stacks are
concentrated in the plus 2 micron portion of soft kaolin crudes. See, for
example, the Morris et al. patent, supra and U.S. Pat. No. 3,743,190,
Whitley. To accomplish "delamination" of these booklets, comminution of
kaolin is carried out under carefully controlled conditions of intensity.
The intent of delamination is to provide impact energy which is just
sufficient to cleave apart the kaolin platelets comprising the booklets
without further fracturing these platelets. The resulting delaminated
particles are highly crystalline. The intent in grinding as practiced in
the present invention is to achieve a desired degree of comminution with
the generation of the highest yield of particles in the desired (finer)
particle size range i.e., 1 micron and finer. The difference is most
readily apparent by comparing the generation of particles finer than 2
microns (as determined by conventional sedimentation techniques). In
delamination, the increase in the content of particles finer than 2
microns particles is generally minimal. In grinding, at least grinding of
the type contemplated in practice of this invention, the increase in the
content of particles finer than 2 microns is significant. It is typically
slightly less than 30% by weight when operating with a degritted crude
(full fraction) and about 15 to 20% by weight when operating with a No. 2
fraction (80% by weight finer than 2 microns). Some media grinding
operations, for example, the glass microballoon media grinding described
in U.S. Pat. No. 3,743,190 (supra) actually effects delamination although
the term "grinding" is used. This is suggested by the text of the patent.
It is well known to kaolin pigment technologists that the amount of iron
and titanium oxides in soft Middle Georgia clays decreases significantly
as the particle size of the clay increases. See also the Gunn et al patent
(supra). In other words, these impurities concentrate in the fine particle
size fractions of soft clays during fractionation. While our process
utilizes substantially all of fine particles (e.g., minus 1 micron
particles in the naturally occurring soft clay) most of the fine particles
used as calciner feed in our process have been typically generated by
grinding coarser kaolin products which have a lower content of colored
impurities than the naturally occurring minus 1 micron fraction of the
crude. Thus, we must grind degritted crude or a fraction of degritted
crude that is low in iron, e.g., less than 0.6% by weight, in order to
assure low iron in the recovered calciner feed. Preferably, the degritted
crude or fraction used as grinder feed in our process is beneficiated by
froth flotation or the like to further remove the undesirable titanium and
iron before grinding. Titanium is especially responsive to removal by
physical/chemical processing such as froth flotation.
The clay is media milled to grind the kaolin particles to a point where a
significant proportion thereof constitutes a suitable size for the
production of fine calcined pigments after minimal removal of oversize by
fractionation. All particle size values referred to herein are determined
by sedimentation using a SEDIGRAPH.RTM. 5100 particle size analyzer.
Typically, the kaolin charge is ground to at least approximately 85% by
weight finer than 2 microns and preferably to at least 90% by weight finer
than 2 microns and then separated by gravity or centrifugal sedimentation
into a fraction which is typically approximately 85% to approximately 95%
by weight finer than 1 micron. The accuracy of measurements at 1 micron
and 2 micron is .+-.2%. Thus, a fraction reported to be 95% finer than 1
micron could be from 93-97% finer than 1 micron. By using a
prefractionated clay as feed to the grinding equipment, as opposed to a
degritted but unfractionated feed, improved recovery of particles finer
than 2 micron is achieved in the classification step following grinding.
The fine ground fraction is recovered, washed, dispersed, spray dried,
pulverized, calcined and pulverized again. If necessary, high intensity
magnetic separation, flotation or other beneficiation techniques can be
applied to the clay advantageously after grinding, when some internally
lodged impurities are exposed by the grinding.
A suitable kaolin feed is obtained by purifying an unfractionated but
degritted soft kaolin crude (about 50-60% by weight finer than 2 microns)
by froth flotation to remove colored impurities, e.g., by the procedure
described in U.S. Pat. No. 4,492,628, Young et al (the teachings of which
are incorporated herein by cross reference). This procedure is referred to
in the clay industry as TREP.
Flotation processes such as ULTRAFLOTATION, e.g., U.S. Pat. No. 2,990,958,
which operate with prefractionated feed (as opposed to whole,
unfractionated feed useful in TREP) is recommended. The teachings of this
patent are incorporated herein by cross reference. In adapting
ULTRAFLOTATION to the present invention, the degritted crude clay is
fractionated into a typical No. 2 coating clay fraction, e.g., a fraction
that is about 80% by weight finer than 2 microns. The fraction remaining
after removal of the coating fraction can be used elsewhere in a kaolin
refining plant.
High intensity magnetic separators (HIMS units) can be used with floated as
well as unfloated feed clay to remove paramagnetic colored impurities,
preferably before grinding.
The ultrafine hard kaolin used to manufacture ANSILEX 93 and similar
calcined kaolin pigments supplied by other kaolin producers is not
suitable as the sole source of kaolin in practice of the invention. The
fine particle fraction (about 90% by weight finer than 1 micron) derived
from this ultrafine kaolin will not result in a pigment of desired
brightness upon calcination to using presently available kaolin
beneficiation technology. Brightness of about 93-94% is typically
obtained. However, such pigments may have the desired low Einlehner
abrasion. Similarly, grinding of fully calcined high brightness pigments
(i.e., post grinding of previously calcined kaolin pigments) will not
yield a fine calcined kaolin pigment displaying a unique combination of
high brightness and low abrasion with excellent performance in paper
filing and coating.
Grinding is carried out with particulate hard grinding media, and is
preferably conducted in the presence of water. This type of operation is
conventionally referred to as "wet grinding". The clay feed is preferably
placed in the form of an aqueous slip that is sufficiently fluid to be
pumped and transported through the grinding equipment. Typically, the clay
solids during grinding is from 20-25% by weight.
Usually a clay dispersant is employed to provide a slip of such solids that
has useful fluidity in the grinding equipment. Preferred dispersant is
ammonium polyacrylate, but sodium polyacrylate or other organic
dispersants employed in kaolin pigment processing can also be used. The
amount of dispersant used is typically 0.05% to 0.10% based on the weight
of clay. An excess of dispersant beyond that needed for good dispersion
may be added at the beginning of the milling period (or in the grinder
feed, if continuous operation is used) to allow for the newly developed
surface area during the grinding operation.
The grinding media should be a dense hard particulate material which does
not discolor the clay charge or leave objectionable residues in the ground
clay. Density of the grinding media is preferably at least 2.4. A
preferred medium is glass beads (such as 20-40 mesh). Examples of other
milling media are alumina, zircon, small ceramic balls, coarse sand,
plastic cylinders, beads, or pellets of nylon, styrene-divinyl benzene
copolymer, polyethylene or other plastic. Grinding should take place in
equipment which is not degraded by the grinding media since this may
result in staining of the ground clay charge.
Most preferably, the milling media is minus 20 plus 50 mesh (US sieve)
glass beads. Generally, the volume of beads to clay slurry varies between
20-70%, most preferably between 35% and 50%. The clay feed to the process
should typically be controlled between 20% to 50% solids; however, optimum
processing conditions are often achieved between 35 and 45% solids.
A suitable vessel used for the process contains vertical baffles and
typically has a height to diameter ratio greater than 1.0 and optimally
1.5 to 2.0. Such a vessel is equipped with an agitation system containing
multiple agitator elements attached to a vertical shaft. The number and
spacing of the agitators must be optimized for the specific process
conditions in order to impart the necessary combined shear and percussive
and frictional energy input necessary to overcome the Van der Waals forces
holding individual platelets in a stacked array. Energy input required for
delamination will vary due to differences between crudes, process
conditions, and equipment; typically, requiring 10 to 50 horsepower hour
per ton of clay charged to the delaminators.
Conventional pre and post processing steps such as flotation, selective
flocculation, magnet separation, floc/filtration, bleaching and spray
drying may be employed.
In the following examples, delamination was performed in a standard stirred
tank delaminator using glass beads at solids content between 20 and 30%.
The bead content of the delaminators was 45-50%. Delamination was
performed in a batch system for 30-60 minutes. A slurry of delaminated
kaolin pigments was bleached with a hydrosulfite bleach to meet a
brightness specification and flocced with sulfuric acid (target pH 3.5)
and alum (at 6 lbs/ton of dry clay) for filtration. Filtration in the
following examples was performed using pan filters. The filtercake was
washed and re-dispersed using a blend of soda ash and polyacrylate (C-211)
as a dispersant. This was followed by spray drying.
Grinding time will depend on the specific mechanical details of the
grinding unit (bead size, specific gravity, charge, clay solids, intensity
of agitation, etc.) and the coarseness of the clay feed. Typically
grinding time is 20-60 minutes in the laboratory impeller driver unit. In
a commercial unit, the average retention time is about 40-60 minutes.
The slip of fine particle size ground clay is passed through a centrifuge
to remove particles larger than 2 microns and then optionally further
purified by magnetic separation and brightened by a bleach such as sodium
hydrosulfite (dithionite). The clay is then filtered and washed. The
filter cake is dispersed preferably with ammonium polyacrylate before
spray drying. Prior to conversion to fully calcined kaolin, the ground
clay must be pulverized. Commercial vertical and horizontal rotary
calciners can be used to produce conventional low abrasion calcined kaolin
pigments. Operation is controlled to avoid calcining at sufficiently high
temperatures to form mullite. Suitable calcination temperatures are in the
range of 1950.degree. to 2100.degree. F. (1065.degree.-1150.degree. C.).
In the examples, samples were calcined in open refractory trays in a muffle
furnace at temperatures of 1065.degree.-1150.degree. C. for 52 min. The
trays were rotated 180.degree. after 30 minutes.
After calcination, the material is pulverized. In the laboratory-scale
examples, this was done in a single pass through a MICROPULVERIZER.RTM.
mill equipped with a 0.020" round hole screen. In commercial operation,
this could be accomplished in a HURRICANE.RTM. mill.
The following examples are given for illustrative purposes.
The following test procedures were employed to obtain values reported
herein. The disclosures in the cited patents are incorporated herein by
cross-reference.
G.E. brightness--TAPPI T646 om--86
Black Glass Scatter--U.S. Pat. No. 4,738,726 col.11, lines 34-52
Particle size--Sedimentation using Sedigraph.RTM. 5100 particle size
analyzer--Values reported as microns (equivalent spherical diameter)
Hunter "L" "a" and "b" values were measured using the equipment and
procedures described in U.S. Pat. No. 5,011,534, Young, col. 8, 1. 45-66.
In the Einlehner Abrasion test, the weight loss of a wire disc contacted by
a rotary abrader and test material is used a relative measure of the
abrasiveness of the test material. Details of the procedures and equipment
used to obtain values reported in this application are described in U.S.
Pat. No. 5,393,340 (supra).
EXAMPLE 1
This example illustrates the presently preferred embodiment of the
invention involving the steps of extensive wet grinding of a No. 2
fraction of a flotation beneficiated kaolin clay followed by controlled
removal of plus 2 microns particles from the ground clay to achieve a high
purity ultrafine particle size kaolin suitable as calciner feed to produce
96% brightness calcined kaolin. In a control test representing a method
for producing high brightness calcined clay, outside the scope of the
invention a portion of the same flotation beneficiated clay was
fractionated to about the same particle size distribution and used a
calciner feed without intermediate grinding and fractionation steps. The
example demonstrates both the improvement in product yield and product
quality achieved by practice of the process of the invention.
In these tests, the crude was a soft coarse white kaolin crude from Middle
Georgia. The crude was of the type known to provide a No. 2 coating
fraction responsive to the froth flotation beneficiated process known in
the art as ULTRAFLOTATION.
The crude was degritted in conventional manner in a dragbox using a 40%
solids slurry of crude. The overflow was passed to a product tank. The
degritted crude in the product tank was 60.4% by weight finer than 2
microns and contained 1.78% (wt.) TiO.sub.2 and 0.36% (wt.) Fe.sub.2
O.sub.3. Brightness was 80.1%. Grit content (+325 mesh, US Sieve) was
0.949%. The degritted crude was dispersed with sodium silicate and
fractionated in conventional manner in a centrifuge to a typical No. 2
kaolin cut, i.e., 99.3% finer than 10 microns, 94.2% finer than 5 microns,
76.2% finer than 2 microns and 60.8% finer than 1 micron. The fractionated
dispersed feed was then processed by ULTRAFLOTATION to reduce the
TiO.sub.2 level in conventional manner using a fatty/rosin acid collector,
calcite "carrier" and an alum-silicate hydrosol dispersant at pH 8.1. The
purified kaolin recovered as the underflow product had an unbleached
brightness of 86.3% and contained 0.47% TiO.sub.2 and 0.44 Fe.sub.2
O.sub.3. The flotation treatment removed some of the coarse particles;
consequently, the relative proportion of fines increased. Thus, about 1/3
of the TiO.sub.2 was removed by flotation but Fe.sub.2 O.sub.3
concentration increased because removal of coarse particles resulted in
concentration of fines which were richer in iron. The combined recovery
from the degritting step and the flotation was 60.7% by weight.
PROCESS OF THE INVENTION
In accordance with this invention, a 10 gallon sample of the flotation
purified fractionated kaolin was placed in a delaminator with 0.5#/ton of
C-211 sodium polyacrylate dispersant and 50% bead volume of 0.75 mm glass
beads for one hour. The particle size of the kaolin discharged from the
delaminator was 95.9% finer than 2 microns and 86.8% finer than 1 micron.
Since the feed to the delaminator was about 76.2% finer than 2 microns and
60.8% finer than 1 micron, approximately 20% by weight of additional
particles finer than 2 microns were generated as a result of the grinding
treatment; approximately 26% by weight of additional minus 1 micron clay
was generated during the grinding step.
To convert the flotation beneficiated ground kaolin into suitable calciner
feed, this intermediate product was fractionated in a laboratory
centrifuge under conditions selected to remove substantially all of the
particles larger than 2 microns while maximizing the recovery of particles
finer than 1 micron for use as calciner feed. The resulting fine fraction
(calciner feed) was 97.1% finer than 2 microns and 88% by weight finer
than 1 micron. This fine fraction was recovered at 21.1% solids. The
coarse (discard) fraction was 71.0% minus 2 micron and 41.9% finer than 1
micron. From these data, it was calculated that the recovery of the minus
1 micron particles in the fractionation step was 95.5%. The total
calculated recovery of calciner feed was 57.9% based on the 60.7% weight
recovery of clay after degritting and flotation.
The recovered fine fraction was bleached in a conventional manner
(flocculation with sulfuric acid to pH 3.0 and bleaching with 6#/ton
sodium hydrosulfite). The slurry of bleached kaolin was filtered and
rinsed with water using about one part by weight water to one part by
weight clay. The filter cake was dispersed by adding 5#/ton sodium
polyacrylate at pH 6.2 and spray dried in a conventional manner. The
brightness of the spray dried kaolin was 90.7%.
CONTROL PROCESS
For purposes of comparison, a portion of same flotation beneficiated
fractionated kaolin used in carrying out the process of the invention
(supra) was fractionated in a laboratory centrifuge to a particle size
distribution substantially the same as that of the calciner feed described
above, i.e., 97.9% minus 2 microns and 88.8% minus 1 micron. The coarse
fraction was 38.9% finer than 2 microns and 14.9% finer than 1 micron. In
this test, the flotation beneficiated clay was not subjected to a grinding
step before or after flotation. Thus, all of the minus 2 micron and all of
the minus 1 micron particles in the calciner feed originated in the kaolin
crude whereas in the process of the invention, most of the minus 1 micron
particles and a substantial proportion of the minus 2 micron particles
were produced during grinding.
The recovery for the fractionation step was calculated to be 62%. The total
recovery based on a 60.7% recovery of clay after degritting and flotation
was only 37.6%.
This product was then bleached, filtered and rinsed as described above and
redispersed to pH 6.3 with 5.5 #/ton of the same polyacrylate dispersant
and spray dried.
Table I is a summary of properties of the calciner feeds obtained by the
control process and the process of the invention.
All spray dried samples were pulverized by 3 passes through a
MIKROPULVERIZER.RTM. mill with a 0.020 inch screen. Pulverized products
were calcined in open trays in a muffle furnace at 2000.degree. F. for 52
minutes and repulverized in the same pulverizing mill with the same
screen.
In Table II, properties of the calcined products are compared.
Data in Table II show that the control process for making calcined clay
from a coarse floated clay resulted in a 96.1% brightness, and a 294 black
glass scatter value. These values are acceptable to a papermaker, but this
method resulted in a relatively highly abrasive clay, (26.1 mg Einlehner).
Such an abrasion value is unacceptable to many papermakers. Furthermore,
the clay producer would normally not use this method because it gives a
poor yield, 29.3% compared to a 52.3% yield commonly attained by using a
hard kaolin to obtain a 93 brightness calcined kaolin.
The surprising benefits that were realized as a result of the process of
the invention are that the process of the invention increases the yield to
45.2% and resulted in an Einlehner abrasion which was 20% lower. The 45.2%
yield makes the product acceptable to the clay producer. This process
produces a calcined clay that has a high brightness (96.1%) and acceptable
scatter (283) which when combined with the low Einlehner abrasion of
approximately 20 mg loss produces a product that is valued by paper
customers. Obtaining the high brightness, low Einlehner, acceptable
scatter and high yield are the key factors that make this a viable
commercial process.
TABLE I
__________________________________________________________________________
EFFECT OF PROCESSING ON PROPERTIES OF CALCINED FEED
WT % RECOVERY, BASED ON PSD OF FEED, MICRONS
DEGRITTED
Fe.sub.2 O.sub.3
TiO.sub.2
0.3 0.5 1 2
PROCESS
FRACTION
CRUDE WT %
WT. %
WT. % FINER THAN
__________________________________________________________________________
Control
62 37.6 0.282
0.481
38.8
63.7
88.9
97.9
Invention
95.5 60.7 0.207
0.468
33.6
62.1
88.8
97.1
__________________________________________________________________________
TABLE II
__________________________________________________________________________
COMPARISON OF PROPERTIES OF CALCINED KAOLIN PIGMENTS
POROSITY
BLACK Pore Vol
ABRASION
GLASS cc/g/Pore
PSD OF PRODUCT, MICRONS
GE BRIGHTNESS EINLEHNER
SCATTER
Radius 0.5 0.75 1 2
PROCESS
% % m.sup.2 /g
Angstrom
WT. % FINER THAN
__________________________________________________________________________
Control
96.1 26.1 294 0.975/3000
6.3 -- 49.7
84.4
Invention
96.1 18 283 0.998/3500
4.3 -- 38.2
68
__________________________________________________________________________
EXAMPLE 2
Another possible way to manufacture a high brightness, low abrasion, good
light scattering calcined pigment would be to use a commercially
available, extremely high purity halloysite (a species of kaolin
minerals). A commercial sample of such material having a brightness of
88%, Fe.sub.2 O.sub.3 content of 0.31% and a TiO.sub.2 content of 0.12%
was obtained. This material was pulverized three times through a 0.39 inch
screen and calcined at 1950.degree. F. for various periods of time in a
muffle furnace. After calcination, the material was pulverized (two
passes) through a 0.039 inch screen. Based on the inability of produce
products with low abrasion in previous testing with clay of this origin,
samples were pulverized three passes through the 0.039 inch screen when
carrying out the Einlehner test. Also, the pulverized material was
slurried to 15% solids and then screened through a 100 mesh screen before
testing for Einlehner abrasion. Calcined pigments having 96+% GE
brightness were obtained at calcination time 20 minutes or more, but the
black glass scatter ranged from only 182-204 (too low to be effective);
Einlehner abrasions values ranged from 37.2 to 41.5 mg loss. Clays from
these samples do not have the light scattering capacity and are too
abrasive to be sold in most segments of the paper markets.
Thus, merely calcining a high brightness, high purity kaolin mineral that
is low in iron and low in titania may result in a high brightness calcined
pigment but does not necessarily result in desired opacification and low
abrasion.
EXAMPLE 3
Tests were carried out to determine the results of calcining flotation
beneficiated, ultrafine fractions of kaolin clay without a grinding step.
The ultrafine fractions, subjected to the calcination treatment, had a
particle size distribution similar to the fine fractions processed in the
McConnell et al patent. These fractions were obtained from fine particle
size fractions of kaolin previously purified to remove colored impurities
by TREP or by ULTRAFLOTATION. In the case of kaolin purified by TREP, all
of the kaolin was from a soft Georgia kaolin crude. In the case of kaolin
purified by ULTRAFLOTATION, the feed was a mixture of soft and hard
kaolins.
In both tests, a 90% <2 micron fraction of flotation purified kaolin,
obtained as a dispersed low solids slip, was cut to 90-95% <0.5 micron
using a Merco nozzle bowl centrifuge.
Samples of the ultrafine clay were flocculated with sulfuric acid and
bleached with 10#/t of sodium hydrosulfite.
The bleached pulps were filtered and washed with water (1:1 by weight). The
filter cakes were redispersed with sodium polyacrylate dispersant, spray
dried, pulverized and calcined as in Example 1.
The feeds were as follows: TREP--0.41 wt % Fe.sub.2 O.sub.3, 0.64 wt %
TiO.sub.2 and particle size distribution of 91% <2 micron, 75% <1 micron
and 51% <0.5 micron Ultraflotation--0.70 wt. % Fe.sub.2 O.sub.3, 0.70 wt.
% TiO.sub.2 and particle size distribution of 90% <2 micron, 79% <1 micron
and 61% <0.5 micron.
The results are summarized in Tables III and IV.
TABLE III
__________________________________________________________________________
Effect of Processing and Feed on Properties of Calcined Feed
Wt % Recovery
Based on PSD of Feed (Microns)
Based on
Degritted
% Fe.sub.2 O.sub.3
% TiO.sub.2
0.3
0.5
1 2
Feed Frac.
Crude
wt. wt. Wt. % Finer Than
__________________________________________________________________________
TREP 29.8 12.1 0.42 0.7 62.2
89.3
98.5
99.5
Ultraflotation
95.2 26.2 0.74 0.54 79.9
93.5
97.2
98.4
__________________________________________________________________________
TABLE IV
__________________________________________________________________________
Comparison of Properties of Calcined Kaolin Pigments
Einlehner
Black
Porosity
PSD of Product (Microns)
Brt.
Abrasion,
Glass
Pore Vol/
0.5
0.7 1 2
Feed % mg Scatter
Pore Radius
Wt. % Finer Than
__________________________________________________________________________
TREP 96.9
26.7 344 1.33/2700
19.3
51.6
79.4
95.9
Ultraflotation
95.2
19.1 307 1.22/2199
22.4
49.7
70 87
__________________________________________________________________________
Data in Table IV show that the calcined ultrafine, flotation beneficiated
clay made by the TREP process had high brightness but higher abrasion than
desired. The ULTRAFLOTATION process resulted in a calcined product having
brightness below 95.5% but with low abrasion. However, in both cases,
yields were poor. See data in Table III.
EXAMPLE 4
This example demonstrates that equivalent products can be obtained by
processing of full fraction of floated feed or fractioned floated kaolin
made from the same feed. The processing was essentially the same as the
one described in detail in Example 1. The feed had been beneficiated by
the TREP process. The full fraction product had an initial impurity
content of 0.44% Fe.sub.2 O.sub.3 and 0.46% TiO.sub.2 ; particle size
distribution was 61.3% <2 micron, 48.9% <1 micron, 32.1% <0.5 micron, and
18.9% <0.3 micron. The product obtained by centrifuging the full fraction
in a Bird centrifuge had an impurity content of 0.5% Fe.sub.2 O.sub.3 and
0.5% TiO.sub.2 ; particle size distribution of 80.9% <2 micron, 65% <1
micron, 54,8% <0.7 micron and 43.9% <0.5 micron. Results are summarized in
Tables V and VI.
TABLE V
__________________________________________________________________________
Effect of Processing and Feed on Properties of Calcined Feed
Wt % Recovery
Based on PSD of Feed (Microns)
Based on
Degritted
% Fe.sub.2 O.sub.3
% TiO.sub.2
0.3
0.5
1 2
Feed Frac.
Crude
wt. wt. Wt. % Finer Than
__________________________________________________________________________
Full Frac.
66.4 49.8 0.46 0.47 40.8
67.7
91.8
98.8
Bird Frac.
89.4 45.8 0.5 0.57 36.4
62.1
89.4
98.7
__________________________________________________________________________
TABLE VI
__________________________________________________________________________
Comparison of Properties of Calcined Kaolin Pigments
Einlehner
Porosity
PSD of Product (Microns)
Abrasion,
Pore volume/
0.5 0.7 1 2
Feed Brt.
mg. Pore radius
Wt. % Finer Than
__________________________________________________________________________
Full Frac.
96.5
20.7 1.19/3000
11.2
34.8
65.3
93
Bird Frac.
96.5
21.5 1.23/3100
10.1
33.9
64.9
93.8
__________________________________________________________________________
Using pigments of the invention as a furnish constituent in the manufacture
of uncoated specialty printing papers, products can meet or even exceed
brightness demand and can substantially reduce the cost of raw materials
by replacing some of the pulp fiber in the furnish. Recycled specialty
papers, at least comparable in quality to paper produced from virgin
fibers, can be manufactured. A less expensive grade of recycled pulp can
be used to achieve the same sheet brightness achieved using 93% brightness
calcined kaolin. There is essentially no difference in opacity between the
commercial Ansilex 93 pigment and a 96% brightness pigment of the
invention. However, since opacity is a function of light absorption and
scatter, one would expect the pigment of the invention to have higher
scatter (due to its greater porosity e.g., by approximately 0.100 cc/g and
lower absorption due to higher brightness. The effects of the factors tend
to cancel each other out in paper; thus, paper sheets filled with the 96%
brightness pigment of the invention would have higher brightness with
equivalent opacity as compared to sheets filled with 93 brightness
calcined kaolin.
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